1. Field of the Invention
The present invention relates generally to an apparatus and method for modulating the phase of a polarized beam and more particularly, to an apparatus and method for minimizing or eliminating thermally induced optical distortion in a Kerr Cell by introducing a transversely flowing electro-optically active medium.
2. State of Technology
Laser Q-switching, (i.e., high gain laser switching) may be achieved via acousto-optical (AO) switches, Pockels cells, bleachable crystals and moving mirrors. Bleachable Q-switches operate upon absorption principles and do not scale for high average power laser systems. Mechanical switches are slow transition switches that produce higher losses and longer pulses and have serious tolerance, alignment, and reliability issues. AO devices are capable of being used in high average power Q-switched laser systems but do not hold off high gain values and have much slower transition times than electro-optic switches. This results in long pulses of relatively low pulse energy. Thus, as the average power increases for such systems, residual absorption will eventually manifest itself as spatial variations in the index of refraction and thermally induced birefringence of the AO media. Pockels cells are popular as electro-optic Q-switches but limit oscillators to average powers of 10 Watts. Above this average power, strong thermal-optical lensing effects can destroy laser mode stability and gain stand-off.
Some of the earliest Q-switched lasers were based on liquid Kerr cells. When a non-moving exemplary dielectric medium, such as for example a nitrobenzene liquid, is enclosed in a cell, i.e., a container that is capable of passing an optical beam, and is subjected to a strong dc field by the use of a pair of electrodes and if an electromagnetic traveling wave is introduced into the medium, the plate field can have an effect on the index of refraction of the medium as seen by the traveling wave. This effect, known as the Kerr Effect, causes an effective birefringence in the liquid medium with the principle axis parallel and perpendicular to the plates and therefore induces polarization rotation in the traveling wave if the waves initial polarization is 45 degrees to the plate orientation. If such a device is put inside a laser cavity containing a polarizing element at 45 degrees to the plates and the plate voltage is switched, a q-switched laser pulse is produced. If such a cell is placed between predetermined polarizers, the cell coupled with the polarizers, i.e., a Kerr Modulator, is capable of amplitude modulation or beam shuttering or beam direction switching. If the traveling wave is polarized along an induced principle axis and a modulated voltage is applied to the plates, then the traveling wave will be phase modulated. If for this last case, a large DC voltage is applied to the plates as well as a smaller modulated voltage, then the phase modulation of the traveling wave will be linear with the modulated drive voltage. Though such a system has benefits such as ease of alignment, since with the fields off, it is not a birefringent medium, as are the solid state Pockels Cell crystals, low average power beam users prefer not handling nitrobenzene and worrying about seal leaks. However, such static and near static (i.e., in some cases low flow is used for filtering), liquid devices are capable of high laser oscillator gain stand-off, and various modulations of low average power light such as ultra fast shutters. Moreover, for both solid state and static liquid state electro-optical devices, high average power optical loads (e.g., up to a few hundred watts in advanced designs) induce thermo-optical distortions that are unacceptable for operation of such devices.
Accordingly, the present invention provides a solution for eliminating or minimizing thermo-optic effects, e.g., induced optical distortion, in Kerr cells to produce a simple, high gain, high average power Q-switch that is additionally capable of direct phase modulation of high average power beams.
Accordingly, the present invention provides an optical apparatus that is capable of minimizing thermally induced optical distortion by moving an electro-optically active medium into and out of a container via a circulation system.
Another aspect of the present invention is to provide an optical apparatus having at least one container arranged along an optic axis that is capable minimizing thermally induced optical distortion by transversely flowing an electro-optically active medium into and out of each of the containers via a circulation system.
Another aspect of the present invention is to provide at least a first and a second container arranged along an optic axis and adapted to receive and pass an input optical beam with a predetermined polarization. The apparatus includes at least one pair of electrodes positioned in each of the containers and arranged substantially parallel to the optic axis, a fluid introduced within the containers that is capable of electro-optical activity when subjected to the electrodes interposed electric field and a circulation system to produce a transverse flow of the fluid into an out of each container with respect to the input beam and adapted to counter-flow the fluid into and out of the second container with respect to the first container such that thermally induced phase errors are further minimized.
Another aspect of the present invention is to provide an optical method for: directing a beam having a predetermined polarization and direction to at least one container arranged along an optic axis and adapted to receive and pass the beam, transversely flowing a fluid capable of electro-optical activity into and out of each of the containers to minimize thermally induced optical distortion; and applying a predetermined electric field to the fluid in each container such that the liquid becomes electro-optically active.
Accordingly, the invention provides a high average power, high gain stand off q-switch that is beneficial in simplifying present high power pulsed laser systems and permitting the development of much larger high power pulsed laser systems. In addition, the present invention can be arranged external to a laser system and operated as an optical switch that is capable shuttering or redirecting very high power levels. The invention can also directly phase modulate an external beam to provide a fast polarization rotator at high average power levels or it can be configured to amplitude modulate the beam. Such a device is beneficial for material processing applications such as cutting, drilling, and laser shock peening.